In radiosurgery or radiotherapy (collectively referred to as radiation treatment) very intense and precisely collimated doses of radiation are delivered to the target region in the body of a patient in order to treat or destroy lesions. Typically, the target region is comprised of a volume of tumorous tissue. Radiation treatment requires an accurate spatial localization of the targeted lesions. Stereotactic radiosurgery (SRS) is a specific type of image-based treatment, which delivers a high dose of radiation during a single session. Because a single radiosurgery dose is more damaging than multiple fractionated doses, the target area must be precisely located.
In general, radiation treatments consist of several phases. First, a precise three-dimensional (3D) map of the anatomical structures in the area of interest (head, body, etc.) is constructed using any one of (or combinations thereof) a computed tomography (CT), cone-beam computed tomography (CBCT), magnetic resonance imaging (MRI), positron emission tomography (PET), 3D rotational angiography (3DRA), or ultrasound techniques. This determines the exact coordinates of the target within the anatomical structure, namely, locates the tumor or abnormality within the body and defines its exact shape and size. Second, a motion path for the radiation beam is computed to deliver a dose distribution that the surgeon finds acceptable, taking into account a variety of medical constraints. During this phase, a team of specialists develop a treatment plan using special computer software to optimally irradiate the tumor and minimize dose to the surrounding normal tissue by designing beams of radiation to converge on the target area from different angles and planes. Third, the radiation treatment plan is executed. During this phase, the radiation dose is delivered to the patient according to the prescribed treatment plan.
The objective of radiation therapy is to accomplish tumor control while sparing the normal tissue from radiation induced complications. This, however, requires an exact knowledge of the target position (tumor position) not only at the planning stage but also the actual treatment times. Conventionally, the tumor position is determined at one single time during the treatment planning. This information may not, however, be accurate during treatment delivery due to patient setup errors, organ motion, and variations of the geometric parameters of the system.
The prevalence of target-conforming beams, as well as the movement toward hypofractionation and dynamic arc IMRT, increases the need for accurate target positioning. Image guidance provides an improvement in positioning accuracy. Image guidance involves acquiring setup images, such as kV radiographs and/or MV portal images from multiple gantry angles, or room based X-ray systems, or MV and/or kV Cone Beam CT, as well as in-room spiral CT's or MRI images to help target localization. If gantry and imager rotation about the isocenter would be completely rigid and planar, the target positions determined from images from multiple angles would be accurate. However, gantry and imager rotation about the isocenter is not rigid and planar. Instead, a gantry head sag imposed by the weight of the gantry head, as well as similar sags in the supports for the MV image panel, kV image panel, and kV source contribute to displacements of the imaging axis and the radiation beam axis from the isocenter. Therefore, a target position determined from images obtained at multiple gantry angles could be offset significantly. In order to compensate for these offsets, the deviations between the treatment beam axis and the imaging axis need to be determined for all gantry angles and the deviations corrected.
Using a treatment couch to compensate for such deviations requires a high precision treatment couch, especially for high precision treatments such as stereotactic radiosurgery and stereotactic body radiation. Treatment couches, however, have mechanical weaknesses which, if not corrected, introduce errors in the accurate positioning and localization of the target. The currently available couch compensation models that correct for mechanical weaknesses, such as load dependent deflections, of the radiation treatment couches are couch dependent, and do not correct for installation variations or readout system production variations.